The use of wireless communication devices such as telephones, pagers, personal digital assistants, laptop computers, etc., hereinafter referred to collectively as “mobile appliances” or “mobiles”, has become prevalent in today's society. Recently, at the urging of public safety groups, there has been increased interest in technology which can determine the geographic position, or “geo-locate” a mobile appliance in certain circumstances.
In the United States, mobile wireless appliance locating equipment is being deployed for the purpose of locating wireless callers who dial 911. Other services in addition to emergency call servicing are contemplated and are referred to as location based services (LBS). Wireless location equipment is typically employed as an overlay to wireless communication networks, thus forming a network overlay geo-location system.
In operation, these network overlay location systems take measurements on radio frequency (RF) transmissions from mobile appliances at base station locations surrounding the mobile appliance, and estimate the location of the mobile appliance with respect to the base stations. Because the geographic location of the base stations is known, the determination of the location of the mobile appliance with respect to the base station permits the geographic location of the mobile appliance to be determined. The RF measurements of the transmitted signal at the base stations may include the time of arrival, the angle of arrival, the signal power, or the unique/repeatable radio propagation path (radio fingerprinting) derivable features. In addition, the geo-location systems may also use collateral information, e.g., information other than that derived for the RF measurement to assist in the geo-location of the mobile appliance, i.e., location of roads, dead-reckoning, topography, map matching, etc.
In a network-based geo-location system, the mobile appliance to be located is typically identified and radio channel assignments determined by (a) monitoring the control information transmitted on radio channel for telephone calls being placed by the mobile appliance or on a wire line interface to detect calls of interest, i.e., 911, and (b) a location request provided by a non-mobile appliance source, i.e., an enhanced services provider. Once a mobile appliance to be located has been identified and radio channel assignments determined, the location determining system is first tasked to determine the geo-location of the mobile appliance and then directed to report a determined position to the requesting entity or enhanced services provider.
The monitoring of the RF transmissions from the mobile appliance or wire line interfaces to identify calls of interest is known as “tipping”, and generally involves recognizing a call of interest being made from a mobile appliance and collecting the call setup information. Once the mobile appliance is identified and the call setup information is collected, the location determining system may be tasked to geo-locate the mobile appliance.
FIG. 1 shows a conventional mobile-appliance communication system having base stations 10a-c for communicating with a mobile appliance 20. Each base station 10 contains signal processing equipment and an antenna for transmitting to and receiving signals from the mobile appliance 20 as well as other base stations. A Base Station Controller (“BSC”) and/or Mobile Switching Center (“MSC”) 45 is connected to each base station 10 through wireline connection 41. A mobile appliance location determining sensor 30, i.e., wireless location sensor (“WLS”), may be positioned at some or all of the base stations 10 to determine the location of mobile appliance 20 within the signal coverage area of the communication system. A network overlay system is generally composed of two main components, one that resides at the base station that makes measurements on the RF signal emanating from the wireless device, the WLS 30, and one that resides at the mobile switch that tasks the WLS groups to collect data and then uses the data to compute a location estimate. This latter component is generally referred to as the Geolocation Control System (“GCS”) 50.
In the normal course of operation, the GCS is tasked by an outside entity, e.g., the Mobile Positioning Center (“MPC”) 40, to generate a location estimate on a particular mobile appliance. The tasking is accompanied by information on the mobile of interest including the serving base station and sector for the call and the RE channel (frequency, time slot, CDMA code, etc.) being used by the wireless communications network to complete the wireless connection. Once the GCS receives this tasking, based on the serving sector, it tasks a set of WLS units to make measurements on the RF emissions of the mobile. The WLS units make the measurements, and report them to the GCS. The GCS then computes a location estimate using a mathematical or data matching algorithm. Alternatively, control signaling on RF or wireline interfaces used to set up calls in the wireless network may be scanned to detect the placement of a call of interest. The signaling that occurs on the RF control channel may be used to determine location, or call setup/channel assignment parameters may be extracted from the control massaging to determine which traffic channel to use for location related measurements.
Network overlay location systems typically locate a mobile appliance on the traffic channels of a wireless network. The system typically uses sensors employing techniques of Time Difference of Arrival (“TDOA”) supplemented with Angle of Arrival (“AOA”) in some cases to perform a multi-site location computation. The traffic channel assignment information is provided through a separate process, with one option being a wireline interface providing MOBINFO (IS-41 Mobile Information) parameters passed by the Mobile Positioning Center 40 as part of the GPOSREQ (J-STD-036 Geolocation Position Request) message from the MPC 40 to the GCS 50.
A network overlay system is generally composed of two main components, one component which resides at the base station that makes measurements on an RF signal emanating from a wireless device with the WLS 30, and the other component which resides at a mobile switch that tasks the geo-location sensor groups to collect data and then uses the data to compute a location estimate. This component is generally referred to as the Geo-location Control System 50 (“GCS”).
Techniques used to locate (AOA, TDOA, etc.) are all described in prior art. One facet of their operation that is important is the process whereby one site, the WLS co-located with the serving base station, is designated as the primary, and it sends information bits relating to the sample of the received signal to the other sites designated as the secondary sites to assist the secondary sites in increasing hearability of the signal of interest to make location related measurements. Various methods have been developed to define and coordinate the tasking, detection and reporting functions. One such method is described in U.S. Pat. No. 5,327,144 to Stilp which is hereby incorporated by reference.
In general, network overlay geolocation systems are deployed at virtually all base station sites to achieve a desired accuracy. A specific problem in prior art methods is that a primary wireless location sensor must be located at the serving base station. Because of cost and other reasons there is now a desire to put WLS equipment into a subset of the base station sites (sparse network deployment) and still maintain high location accuracy.
In general, network overlay location systems use location related measurements from many sites to estimate the location. For example, in GSM systems typically 6 or 7 sites participate in the location estimate. One effect of not having a WLS at every site is degradation of location accuracy. For some air interfaces, this may be problematic. For example, in AMPS and TDMA where the occupied bandwidth is small, the error associated with location estimates where the number of participating sites is reduced becomes unsatisfactory. Air interfaces such as GSM and CDMA do not suffer from this, and mobile appliances operating in these air interfaces are wider bandwidth and/or frequency hopped. These features allow the TDOA or AOA surfaces generated to be less corrupted by multi-path (multi-path can be better resolved in the time domain), and therefore, even with fewer surfaces, the location estimates are generally acceptable.
Another effect of the sparse overlay is “no location areas”. “No location areas” are those areas in which a minimum number of WLS cannot detect or measure an attribute of a signal such that the geo-location system cannot estimate a location. Mobile appliances are power controlled by the wireless network. This means that the mobile's transmit power is changed by the network so that that minimum power is transmitted to achieve an acceptable communications link (i.e., the voice quality is acceptable). When a mobile appliance moves close to a base station site, the required transmit power for an acceptable communications link is reduced to a small value. This power control is well known in the art and is desirable because it diminishes co-channel interference and adjacent cell interference where channel reuse is employed and prolongs the battery life of the mobile appliance. However, if the base station that is serving the mobile appliance does not have WLS (due to the sparse deployment), then there is no WLS to “hear” the mobile at the serving site, and the neighboring site WLS units may not be able to hear the mobile because of severe power control resulting in low transmit power. The result is of this phenomenon is a series of “no location areas” surrounding the base station sites without WLS equipment due to the sparse overlay.
Therefore in order to obviate the deficiencies of the prior art it is an object of the present disclosure to provide a novel method for geo-location in “no location” areas.
It is also an object of the present disclosure to provide a novel method of detecting the target signal independently of a primary WLS.
These objects and other advantages of the disclosed subject matter will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments.